1. Technical Field
The present invention relates to a nonlinear optical element. The present invention particularly relates to a wavelength converting element used for light wavelength conversion.
2. Background Art
Recently, attention has been paid to a high output laser light source as a light source used for processing applications, laser displays, or the like. Solid lasers such as a YAG laser, fiber lasers using fibers having rare earth elements such as Yb and Nd, and the like doped thereto have been developed as laser light sources in the infrared light region. On the other hand, in laser light sources of the visible light region, particularly laser light sources of the red and blue light regions, semiconductor lasers using gallium arsenide, gallium nitride, and so forth have been developed and their high output performance has also been studied. However, it is still difficult to directly generate green light from semiconductor lasers. Because of this, a method is generally used that involves: using the above-mentioned solid laser or the fiber laser to firstly obtain an infrared light laser; and then passing the infrared light through a wavelength converting element of a nonlinear optical crystal to carry out wavelength conversion; and thereby obtaining a green light laser.
Prior to the above-mentioned semiconductor laser being developed, there was almost no method of generating laser light of the ultraviolet region from the visible light region except the wavelength conversion which uses a nonlinear optical crystal. Under such technical backgrounds, a variety of nonlinear optical materials such as lithium niobate (LiNbO3), lithium tantalate (LiTaO3), lithium triborate (LiB3O5), beta-barium borate (beta-BaB2O4), potassium titanyl phosphate (KTiOPO4: KTP) and cesium lithium borate (CsLiB6O10: CLBO) have been actively developed and utilized.
Among the plurality of optical materials taken up as examples above, particularly, it is known that the lithium niobate crystal has a large nonlinear optical constant. Because of its large nonlinear optical constant, a nonlinear optical crystal containing a lithium niobate crystal exhibits high conversion efficiency, and further an apparatus using this crystal is capable of being simply constructed. Thus, a quasi phase matching (QPM) wavelength converting element which is formed with employing a polarization inversion technique to a lithium niobate crystal is frequently used in an apparatus having an output of a level of one hundred mW.
For example, the quasi phase matching (QPM) lithium niobate element (QPM-LN element) using a lithium niobate (LN) crystal or lithium tantalate (LT) crystal has a larger nonlinear optical constant than an LBO crystal or a KTP crystal. Accordingly, the wavelength conversion of high efficiency and high output is possible. However, the QPM-LN element requires condensing light energy in a narrow region. As such, substantially, its crystal breakage and deterioration caused by a fundamental wave and its second harmonic that generates from a fundamental wave within a crystal are liable to occur as compared with the crystal breakage and deterioration of the KTP crystal and the like.
In addition, an apparatus having an output of a level of a few W uses a nonlinear optical crystal such as lithium triborate (LBO) and potassium titanyl phosphate (KTP). The former LBO crystal has the advantageous characteristic that crystal breakage and deterioration caused by a fundamental wave and its second harmonic which generates from the fundamental wave within the crystal hardly occurs. However, the nonlinear optical constant of the crystal is small. Because of this, for obtaining high conversion efficiency, a resonator is configured, and the crystal is placed therein. Thus, the configuration of the apparatus becomes complicated and the apparatus also has the disadvantage needing fine adjustment. The nonlinear optical constant of the latter KTP crystal is large as compared with that of the LBO crystal. Hence, the KTP crystal, not like the LBO crystal, can obtain high conversion efficiency without a resonator. However, the KTP crystal has the disadvantage that the crystal breakage and deterioration caused by a fundamental wave and its second harmonic which generates from the fundamental wave within the crystal is liable to occur.
The above-mentioned crystal deterioration includes a refraction index change caused by light (photorefractive). Conventionally, for the restraint of photorefractive, which is one of the crystal deteriorations, impurities which generate absorption peaks within the crystal are removed as much as possible; for the purpose of compensation of holes generated even when that removal is done or of a charge generated by an anti-site defect, which is a presence of different element at the site where originally another element constituting the crystal has existed, the control of shifting the absorption end of transmittance towards the shorter wavelength region by the doping of magnesium oxide or zinc oxide, or the improvement of the transmittance of the visible region is generally performed.
JP 06-016500 A (JP '500) tries the restraint of index change caused by light (photorefractive) by introducing an additive into a crystal (lithium niobate (LN) and lithium tantalate (LT)), and JP 2002-072266 A (JP '266) also tries the restraint by growing a crystal by use of a method of being capable of approaching a crystal composition to an ideal composition (stoichiometry: stoichiometric composition).
However, at present, even those attempts in the above JP '500 and JP '266 have not completely restrained the crystal destruction and deterioration.
As mentioned above, the nonlinear optical crystals each has an advantage and disadvantage. At present where those disadvantages have not been successfully constrained, the advantage and disadvantage of the nonlinear optical crystal are in a relation of trade-off. Because of this, at present, we have to determine the crystal to be used according to the applications while taking into consideration the relation of such trade-off.
Additionally, JP 11-271823 A (JP '823) discloses a wavelength converter having a plurality of wavelength converting elements. FIG. 1 is a schematic block diagram of the wavelength converter described in JP '823. In this wavelength converter, power densities of a fundamental wave being input to each of two wavelength converting elements 102a, 102b are restrained to be low to suppress the deterioration of a wavelength converting element and also the conversion efficiency of the entire wavelength converter is improved. However, a wavelength converter having such as an apparatus configuration includes problems such as a high production cost, complexity of apparatus adjustment, and so forth.